Msm Arthritis

Elderly pony with arthritis....?
Ok, here's the thing, I have a 17 yr old exmoor type pony, so shes around 13hh she has quite bad arthritis in one knee and shoulder!!
We usually give her "one" of the following, along with oils and devils relief (devils claw): MSM, cortaflex or Glucosamine.
This summer we bought all 3! I'm just wondering if its safe to give her all 3 together? or at least 2 of them...
we have:-100% MSM
- Cortaflex
- Glucosamine 10,000 plus MSM
So i'm thinking the glucosamine already has MSM in it..? and i don't want to over do it! I would obviously be cutting the recommended amount 2 take in each by 3rd if i was 2 give her 2 or all 3 of them but i don't know if I should just stick to the usual and give her one at a time!?
Please help, i know i probably am not making alota sense....?
Thanks
seems like you have tried everything.....my sisters horse is 28...she has a bit of arthritis too..... along with cortaflex she gives her danylon....you have to get it from vets......but it keeps her sound enough to ride lightly.....